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  <p class="align-right"><strong>built-in method</strong></p>
<section id="parameter-2">
<span id="methods-parameter-2"></span><span id="index-0"></span><h1><code class="docutils literal notranslate"><span class="pre">parameter/2</span></code><a class="headerlink" href="#parameter-2" title="Link to this heading"></a></h1>
<section id="description">
<h2>Description<a class="headerlink" href="#description" title="Link to this heading"></a></h2>
<div class="highlight-logtalk notranslate"><div class="highlight"><pre><span></span><span class="k">parameter</span>(<span class="nv">Number</span>, <span class="nv">Term</span>)
</pre></div>
</div>
<p>Used in <a class="reference internal" href="../../glossary.html#term-parametric-entity"><span class="xref std std-term">parametric objects</span></a> (and
parametric categories), this private method provides runtime access to
the parameter values of the entity that contains the predicate clause
whose body is being executed by using the argument number in the entity
identifier. This predicate is implemented as a unification between its
second argument and the corresponding implicit execution-context
argument in the predicate clause making the call. This unification
occurs at the clause head when the second argument is not instantiated
(the most common case). When the second argument is instantiated, the
unification must be delayed to runtime and thus occurs at the clause
body.</p>
<p>Entity parameters can also be accessed using <em>parameter variables</em>,
which use the syntax <code class="docutils literal notranslate"><span class="pre">_VariableName_</span></code>. The compiler recognizes
occurrences of these variables in directives and clauses. Parameter
variables allow us to abstract parameter positions, thus simplifying
code maintenance.</p>
</section>
<section id="modes-and-number-of-proofs">
<h2>Modes and number of proofs<a class="headerlink" href="#modes-and-number-of-proofs" title="Link to this heading"></a></h2>
<div class="highlight-logtalk notranslate"><div class="highlight"><pre><span></span><span class="k">parameter</span>(<span class="o">+</span>integer, <span class="o">?</span>term) <span class="o">-</span> zero_or_one
</pre></div>
</div>
</section>
<section id="errors">
<h2>Errors<a class="headerlink" href="#errors" title="Link to this heading"></a></h2>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">Number</span></code> is a variable:</div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">instantiation_error</span></code></div>
</div>
<div class="line"><code class="docutils literal notranslate"><span class="pre">Number</span></code> is neither a variable nor an integer value:</div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">type_error(integer,</span> <span class="pre">Number)</span></code></div>
</div>
<div class="line"><code class="docutils literal notranslate"><span class="pre">Number</span></code> is smaller than one or greater than the parametric entity identifier arity:</div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">domain_error(out_of_range,</span> <span class="pre">Number)</span></code></div>
</div>
<div class="line"><code class="docutils literal notranslate"><span class="pre">Entity</span></code> identifier is not a compound term:</div>
<div class="line-block">
<div class="line"><code class="docutils literal notranslate"><span class="pre">type_error(compound,</span> <span class="pre">Entity)</span></code></div>
</div>
</div>
</section>
<section id="examples">
<h2>Examples<a class="headerlink" href="#examples" title="Link to this heading"></a></h2>
<div class="highlight-logtalk notranslate"><div class="highlight"><pre><span></span><span class="p">:- </span><span class="k">object</span>(box(<span class="nv">_Color</span>, <span class="nv">_Weight</span>)).

    ...

    <span class="c">% this clause is translated into</span>
    <span class="c">% a fact upon compilation</span>
    color(<span class="nv">Color</span>) <span class="o">:-</span>
        <span class="k">parameter</span>(<span class="m">1</span>, <span class="nv">Color</span>).

    <span class="c">% upon compilation, the &gt;/2 call will be</span>
    <span class="c">% the single goal in the clause body</span>
    heavy <span class="o">:-</span>
        <span class="k">parameter</span>(<span class="m">2</span>, <span class="nv">Weight</span>),
        <span class="nv">Weight</span> <span class="o">&gt;</span> <span class="m">10.</span>
    ...
</pre></div>
</div>
<p>The same example using <em>parameter variables</em>:</p>
<div class="highlight-logtalk notranslate"><div class="highlight"><pre><span></span><span class="p">:- </span><span class="k">object</span>(box(<span class="nv">_Color_</span>, <span class="nv">_Weight_</span>)).

    ...

    color(<span class="nv">_Color_</span>).

    heavy <span class="o">:-</span>
        <span class="nv">_Weight_</span> <span class="o">&gt;</span> <span class="m">10.</span>

    ...
</pre></div>
</div>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><a class="reference internal" href="context_1.html#methods-context-1"><span class="std std-ref">context/1</span></a>,
<a class="reference internal" href="self_1.html#methods-self-1"><span class="std std-ref">self/1</span></a>,
<a class="reference internal" href="sender_1.html#methods-sender-1"><span class="std std-ref">sender/1</span></a>,
<a class="reference internal" href="this_1.html#methods-this-1"><span class="std std-ref">this/1</span></a></p>
</div>
</section>
</section>


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